Superabsorbents are known, the term designating crosslinked hydrophilic polymers capable of imbibing large amounts of aqueous fluids. This capability rests on the strong interaction of water with hydrophilic groups on the superabsorbents, in particular ionic groups or groups capable of hydrogen bonding. Other customary designations for what are known as superabsorbents include “superabsorbent polymer”, “hydrogel” (often even used for the dry form), “hydrogel-forming polymer”, “water-absorbing polymer”, “absorbent gel-forming material”, “swellable resin”, “water-absorbing resin” or the like. Water-absorbing polymers based on partially neutralized acrylic acid are concerned in particular. The essential properties of superabsorbents are their ability to absorb a multiple (30-800 times for example) of their own weight of aqueous fluids and to retain the fluid even under some pressure. The superabsorbent, which is used in the form of a dry powder, turns into a gel on imbibing liquid and so turns into a hydrogel when as usual imbibing water. Crosslinking is essential for synthetic superabsorbents and renders the polymers insoluble in water. Soluble substances would not be useful as superabsorbents. By far the most important field of use for superabsorbents is that of absorbing bodily fluids. Superabsorbents are used, for example, in diapers for infants, incontinence products for adults or femcare products. Fields of use further include, for example, as a water-retaining agent in market gardening, as a water store for protection against fire, for fluid absorption in food packaging, as cable cladding material for deep sea cables or, very generally, for absorption of moisture.
Such a superabsorbent in general has a CRC (“Centrifuge Retention Capacity”) of at least 5 g/g, preferably at least 10 g/g, more preferably at least 20 g/g, especially 30 g/g. It is not just its absorption capacity which is important for a superabsorbent, but also its ability to retain liquid under pressure, usually expressed as “Absorption against Pressure” (“AAP”) and also its permeability, i.e. the ability to conduct liquid in the swollen state. Flow conductivity to as yet unswollen superabsorbent may be blocked by swollen gel (“gel blocking”). Good conductivity properties for liquids are shown, for example, by hydrogels that have a high level of gel strength in the swollen state. Gels having only low gel strength are deformable under an applied pressure (body pressure), cause pores to collapse in a superabsorbent/cellulose fiber pad and thereby block flow conductivity to as yet unswollen or incompletely swollen superabsorbent and the imbibition of liquid by this, as yet unswollen or incompletely swollen superabsorbent. Elevated gel strength is generally achieved through a relatively high level of crosslinking, but this reduces the absorption capacity of the product. An effective method of increasing gel strength is to increase the level of crosslinking at the surface of the superabsorbent particles compared to the interior of the particles. To this end, superabsorbent particles which have usually been dried and have an average crosslink density are subjected to additional crosslinking in a thin surface layer of the particles thereof. Surface postcrosslinking increases the crosslink density in the shell of the superabsorbent particles, raising the absorption under confining pressure to a higher level. While the absorption capacity in the surface layer of the superabsorbent particles decreases, the presence of mobile chains of polymer in their core leads to an improved absorption capacity compared with the shell, so shell construction ensures an improved permeability without occurrence of gel blocking. It is likewise known to produce comparatively highly crosslinked superabsorbent overall and to subsequently reduce the degree of crosslinking in the interior of the particles versus an outer shell of the particles.
The manufacture of such superabsorbents (or superabsorbent polymers) is based essentially on the polymerization of ethylenically unsaturated acid-functional monomers which are optionally at least partly present as a salt, in particular on the free-radical polymerization of partially neutralized acrylic acid, typically in the presence of crosslinkers.
When additive treatment of the polymer is required during the production of such superabsorbents, for which it is customary to use liquid preparations of the additives to be added, in particular aqueous solutions of additive. These liquid preparations, in particular solutions, of additives are typically applied to the polymer by nozzle spraying in order to obtain a particularly good distribution. Nozzle plugging in particular is one of the frequent issues here ranging from loss of product quality to costly and inconvenient maintenance being required to production outages. Heatable nozzles etc. are therefore used for example.
The problem addressed by the present invention against this background was specifically that of improving in the manufacture of water-absorbing polymeric particles the manner of adding liquid additive preparations to the effect that issues ranging from loss of product quality to production outages due to the admixture of additive become better avoidable.
It was found that, surprisingly, this problem is solved by the subject matter of the invention, namely. a process for producing water-absorbing polymers wherein a base polymer A is contacted and mixed with a liquid additive preparation B, in particular an aqueous additive solution B, in a mixing device, wherein the base polymer A is formed from monomers bearing at least 30%, preferably at least 40%, neutralized acid groups and the liquid additive preparation B, in particular the aqueous additive solution B, is dosed into the mixing device without nozzle via a pipe.
This process makes possible an essentially trouble- and maintenance-free admixture of liquid additive preparations in the manufacture of water-absorbing polymeric particles. Surprisingly, product quality is not compromised despite the eschewal of nozzles for spray dispensing the liquid additive preparations. Production outages become avoidable.
Further advantages of the invention are that maintenance requirements are very significantly reducible overall since, for example, there is much less fouling and hence cleaning requirements are appreciably reduced. Reduced residence time in the mixer is also made possible. Aerosol formation is avoided. Mixers can be used more efficiently. All this results in a distinct reduction in manufacturing costs.